1. Field of the Invention
The present invention relates to a method for manufacturing lepidocrocite (.gamma.-FeOOH). More specifically, it relates to a method for manufacturing lepidocrocite suitable as a starting material for the manufacture of magnetic powders such as magnetic iron oxide powders and magnetic metal iron powders for magnetic recording media (audio tapes, magnetic disks video tapes, and magnetic cards).
2. Description of the Related Art
Generally, magnetic iron oxide powders for magnetic recording media such as audio tapes, video tapes, magnetic disks and magnetic cards are produced from goethite (.alpha.-FeOOH) or lepidocrocite (.gamma.-FeOOH) as a starting material. .alpha.-FeOOH or .gamma.-FeOOH is subjected to the treatments such as calcination (for dehydration and inter-sintering), reduction, and to obtain maghemite (.gamma.-Fe.sub.2 O.sub.3) which are needle-shaped (acicular) magnetic iron oxide powders. The .gamma.-Fe.sub.2 O.sub.3 powders may be subjected to a further treatment of cobalt modification, to provide cobalt-coated .gamma.-Fe.sub.2 O.sub.3 powders(Co-.gamma.-Fe.sub.2 O.sub.3). Further, acicular .alpha.-FeOOH or .gamma.-FeOOH is subject to the treatments such as hydrogen gas reduction, while maintaining the acicular shape of the starting material to produce acicular magnetic metal iron powders.
In the above cases, the magnetic properties of the resultant magnetic powders depend on the characteristics of the starting material. Therefore, to obtain magnetic powders suitable for magnetic recording media, it is necessary to use a starting material having narrow particle size distribution, good shape and crystal structure.
Heretofore, magnetic powders produced from magnetic recording media, as the final products, such as an audio tape and a video tape which have good dispersibility and excellent magnetic orientability, squareness ratio, and print-through level but the particle size distribution of the magnetic powders produced from .gamma.-FeOOH is so wide that the switching field distribution of the magnetic powders become high. As a result, at present, .alpha.-FeOOH (goethite) is almost always used as the starting material for the production of .gamma.-Fe.sub.2 O.sub.3.
In the production of goethite (.alpha.-FeOOH), in which an aqueous suspension of ferrous hydroxide is oxidized by blowing an oxygen-containing gas into the suspension, it is considered that the oxidation rate should be matched with the crystal growth speed. To control the oxidation rate, methods are known in which the oxidation reaction of ferrous hydroxide is carried out in several steps defined by a specified percentage of oxidation of ferrous iron in certain time periods respectively (Japanese Unexamined patent publication (Kokai) Nos. 52-59095, 52-59096 and 52-59097, corresponding to German patent application Nos. P 2250225.4, P 2550307.5 and P 2550308.6). Also known are methods in which a part of a suspension of ferrous hydroxide is taken out from a reactor containing the suspension and recirculated to the reactor. The suspension is oxidized during the recirculation, whereby the oxidation rate per one cycle is defined (Japanese Unexamined patent publication (Kokai) Nos. 57-166322, 57-209834, 58-32028 and 58-140327). However, these methods are concerned with the manufacture of goethite, not lepidocrocite.
If the feed rate of an oxygen-containing gas is controlled so that the oxidation rate is matched with the crystal growth speed in the production of lepidocrocite, the control of the feed rate must be very delicate, i.e., the particle size distribution of acicular lepidocrocite is remarkably affected by the delicacy of the control. If a feed rate of an oxygen-containing gas into a suspension for reaction is too high, the reaction rate becomes high to increase the rate of formation of a nucleus, whereby the respective acicular crystal becomes a small acicular crystal with a short length. The particle size distribution of the acicular crystal becomes wide due to the formation of many additional nuclei during the crystal growth. If a feed rate of an oxygen-containing gas is too low, the reaction rate becomes low to decrease the rate of formation of a nucleous, preventing the formation of additional nuclei during the crystal growth, but the respective acicular crystal becomes a large crystal with an extremely long length and raft-like crystal flocks, unique to lepidocrocite, are further aggregated with each other, and the resulting lepidocrocite is an undesirable crystal.